Generally speaking, the method according to the invention allows for the polycrystalline growth of any organic compound comprising molecules having a dipole moment according to an axis of symmetry generally known as a polar axis.
In known electro-optical modulators, monocrystals of mineral materials are generally used, said materials being, for example, potassium diphosphate (KDP), lithium niobate, and lanthanum, lead and zirconium titanate (PLZT).
These mineral materials present the drawback of lacking effectiveness, which requires that they must be used with large thicknesses. Furthermore, the electro-optical effect of these materials owing to their ferroelectricity results in a variation of the crystalline mesh involving high stresses inside the crystals, which generally provokes a separation of the electrodes required for modulation, as well as a premature wear of the material.
In addition, these minerals have a tendency for their refraction index to change in the course of time owing to photorefraction leading to a possible voltage drift of the electro-optical devices comprising these materials.
The manufacture of these monocrystals from mineral materials also requires high temperatures and thus a high energy consumption and significant production stresses.
Also, for a number of years now, research has been centered on the production of monocrystals from organic molecules. These monocrystals have the advantage of offering improved effectiveness as compared with mineral monocrystals, of being able to be produced at relatively low temperatures, and of presenting a wide pass-range of the transparence windows situated in the visible and near-infrared range.
The electro-optical properties of organic monocrystals are due to electronic displacements and not to variations of the crystalline mesh as in the case with mineral crystals, which provides them with improved mechanical behaviour and increases their lifetime.
For more specific details concerning the electro-optical properties of certain organic materials, reference should be made to the article by K. D. SINGER and others which appeared in the publication entitled "Non-Linear Optical Properties of Organic Molecules and Crystals", vol. 1 (1978), pp 437-467, D. Chemla and J. Zyss, Eds., AT&T Labs.
Unfortunately, it is currently extremely difficult to produce large organic material monocrystals able to be used industrially. The methods currently known for manufacturing organic material monocrystals (growth according to the Bridgmann technique or from a vapor) are difficult to implement and in particular can hardly be reproduced.
The difficulty of obtaining large organic crystals is more particularly described in the article by SINGER mentioned above.
The difficulty of obtaining large crystals from an organic material is not a specific property of materials having electro-optical properties. In fact, it is difficult to produce any large monocrystal from an organic material.
Finally, there currently does not exist any reliable commercial electro-optical device as regards organic materials.